The invention relates to a degaussing circuit. The invention also relates to a display apparatus comprising such a degaussing circuit. The invention further relates to a method of degaussing.
Any kind of computer monitor or television apparatus with a colour cathode ray tube (further referred to as CRT) requires a degaussing operation. The degaussing operation minimizes residual magnetic fields in magnetisable parts (for example iron brackets) in or near the CRT. Residual magnetic fields would influence the deflection of the electron beams in the CRT, thereby causing discoloration or wrong colours in information displayed. A degaussing action is performed by generating an AC current through a degaussing coil which is positioned near the CRT. This AC current causes magnetic dipoles in the magnetisable parts to follow the orientation of the magnetic field introduced by the degaussing coil. The number of dipoles which become oriented depends on the intensity of the magnetic field. With a progressive reduction of the absolute value of the AC current, fewer and fewer magnetic dipoles become equally oriented. And finally, when the AC current is reduced to zero, the magnetisable parts are demagnetized.
In a known and cheap way to obtain an AC current with a slowly decreasing amplitude, the AC mains voltage is supplied to a series arrangement of a degaussing coil and a PTC resistor. When the display apparatus is switched on, the PTC resistor is cold and thus has a low resistance. A high inrush of AC current flows through the degaussing coil. The PTC is gradually heated by the AC current, consequently, its resistance gradually increases, and the amplitude of the AC current gradually decreases. Such a degaussing circuit has the drawback that the degaussing action cannot be performed frequently. The PTC resistor has to cool down before a next degaussing action is effective.
DE-A-38.31.306 discloses a degaussing circuit which overcomes this problem. The disclosed degaussing circuit comprises a series arrangement of a degaussing coil and a triac. The series arrangement is coupled to receive an AC mains voltage. A sawtooth voltage is generated during a second half of each half period of the AC mains voltage. A start instant of each sawtooth voltage is generated by detecting a zero crossing of a 90.degree. phase-shifted AC mains voltage. An end instant of each sawtooth voltage is generated by detecting a successive zero crossing of the non-shifted AC mains voltage. The sawtooth voltage is compared with an increasing reference voltage to obtain a control pulse for the triac. The control pulse switches the triac on at the instant the sawtooth voltage crosses the reference level. The triac switches off automatically at the next zero crossing of the AC mains voltage, because then the degaussing current is zero too. The on-time of the triac slowly decreases in successive half periods of the AC mains voltage as the reference level slowly increases. At the start of a degaussing action, a start value of the reference voltage is increased if an amplitude of the AC mains voltage is increased for obtaining a smaller on-time of the triac at the start of the degaussing action.
The disclosed degaussing circuit only compensates for slow amplitude changes of the AC mains voltage. The known degaussing circuit does not compensate for mains frequency deviations or irregularities of the AC mains voltage. Examples of irregularities are fast changing amplitudes such as mains spikes and mains dips, or asymmetrical distortions causing successive half mains periods to have different durations and/or amplitudes.